Any reference to background art herein is not to be construed as an admission that such art constitutes common general knowledge in Australia or elsewhere.
Sealant compositions for pneumatic tires have been developed to provide temporary sealing of pneumatic tires after a puncturing incident has occurred. The sealing of punctured tires with such compositions is a stop gap measure in that it serves the purpose of making the vehicle with the punctured tire drivable again for a limited period of time and/or a limited distance. After that the tire is either to be replaced or, if possible, repaired. Some sealant compositions have been applied prophylactically and become automatically effective the moment damage occurs in the area of the tread of the tire. A disadvantage of this is that the driver of the vehicle with the damaged tire may not be aware the damage to the tire has occurred.
Tire puncture repair kits were developed to conveniently restore vehicle mobility following a puncture event, thereby eliminating the need to carry a spare wheel and associated equipment. This afforded a much quicker, easier and safer means of restoring vehicle mobility as well as saving weight. The kits included a plugin compressor and a canister of latex sealant that could be pumped into the tire without having to remove the valve core. Latex was the only choice initially available and has been used in aerosol sealants for many years. This is because latex was known to have excellent sealing properties including being one of the only adhesive materials that has the ability to bond to itself when it cures.
Latex effectively seals punctures due to a cross linking action caused by the interaction between the latex and the escaping air. The skin forming chemical reaction of latex occurring within the puncture site quickly forms into what would be considered a glued-into-place permanent solid plug. This is contrary to what is the intended outcome as the use of the sealant is meant to be a temporary repair only. Thus, while latex is an ideal material for repairing or sealing punctures, the resultant permanent seal product by the curing of the latex is a major problem. The intended user, the driver of the affected vehicle, is instructed to not exceed 90 km/h and to have the tire checked and repaired or replaced as soon as possible.
Sealant compositions are primarily intended for sealing punctures in the tread area of pneumatic tires. Their effectiveness becomes more problematic when the puncture occurs in the shoulder of the tire. Punctures in side walls of pneumatic tires are not safely sealable, even temporarily, and require urgent replacement of the damaged tire. However, in recent years certain of the original equipment manufacturers (car manufacturers) have escalated their puncture sealing standard to much higher levels. This increased sealing requirement now dictates that a puncture caused by a 6.00 mm diameter spiked object be sealed effectively when the puncture occurs in the extreme outer shoulder edge (virtually in the side wall) and a puncture caused by a 4.00 mm diameter spiked object be sealed effectively when the puncture occurs in the sidewall.
A reliable and effective seal that meets these requirements up to now could only be achieved with a latex formula. Latex is the only sealant that has been accepted to date by the original equipment manufacturers to meet their outer shoulder edge and sidewall sealing requirement. However, latex, due to its permanent sealing capacity, does not permit discrimination between repairable and non-repairable unsafe punctures.
The accepted industry standards require that extreme shoulder edge spiked punctures (i.e. those caused by a spike of 6.00 mm diameter) must pass a 48 hour leak-down test. However, meeting this level of sealing performance contravenes authorised tire repair standards in that permanent repairs are not permitted in the shoulder area or sidewall of a tire. This is due to the high levels of flexing in these regions and the potential risk of structural failure of the reinforced ply cords. Vehicle tires are subjected to extreme flexing and distortion in the outer shoulder edge. Additionally, the tire wall thickness in the outer shoulder edge is less than half that of the tread area, and has far less reinforcement than the tread when one considers the steel belt in the tread. Nevertheless, latex is a close equivalent of a permanent repair. It forms a rubber ‘plug’ that becomes permanently bonded into the puncture site. Hence there is no visual indication that the integrity of the tire's reinforced structure may have been compromised. Most drivers are unaware of the legal restrictions and assume that because a temporarily repaired tire looks inflated, it is therefore automatically deemed safe. This can be a dangerous assumption. If a major intrusion is being camouflaged due to the high level of sealing performance and resilient characteristics of latex, this can lead to sudden air loss escaping from the region of any weakened and fatigued structure.
Another major issue with a conventional latex based sealant composition is the propensity of latex particles, during long-term storage of the composition, to aggregate near the surface of the composition with the result that the composition becomes creamy in appearance with an upper layer of aggregated latex particles. The aggregated particles can clog the outlet of a tire sealant composition container hindering the passage of the sealant composition out of the container, or in passing through the tire valve, into the tire so that a smooth puncture sealing operation cannot be carried out. The disclosure in WO 2007/112010 attempts to manage this problem by incorporating glycerine, or glycerine and potassium acetate, as anti-freeze agents in place of ethylene glycol or propylene glycol, optionally with surfactants. While glycerine is said to be less easily mixed with latex than ethylene glycol, in combination with a surfactant mixing procedure it is said to suppress the upward migration activity of latex particles and their accumulation near the surface of the composition.